1. Field of the Invention
The present invention relates to a radiation image reading apparatus and, more particularly, to a radiation image reading apparatus wherein a storage phosphor plate on which radiation image information is recorded is irradiated with excitation light and the light emitted from the phosphor plate is detected to read out the image information.
2. Description of the Prior Art
Radiation image reading apparatus is known where radiation image information of an object recorded on a storage phosphor plate known as an imaging plate (IP) is scanned with excitation light to generate accelerated light emissions, the light emissions are read out in sequence by a photoelectric means to generate image signals of the image, and the image is displayed by a suitable display apparatus as a visible image or recorded on a film as required. The image information is recorded by any means of radiation image recording techniques.
When using a phosphor plate for recording the radiation image, the required exposure to radiation, such as X-rays and the like, may be lowered by the light sensitivity of the storage phosphor, and the phosphor member may be repetitively used due to the recorded image being erasable, and further the image signals may be easily processed owing to the signals being processed in digital forms.
Such a radiation image reading apparatus according to the prior art is explained by referring to FIG. 1. In FIG. 1, numeral 1 designates a laser beam source, numeral 2 a light deflector such as a polygon mirror, a galvanometer mirror or the like, numeral 3 a plane mirror and numeral 4 a storage phosphor plate. The laser beam emitted from the laser beam source 1 is deflected linearly by means of the light deflector 2 as shown by an arrow a and scans over the phosphor plate 4 in the main scanning direction X by way of the plane mirror 3. Although not shown, the reading apparatus includes means for transferring the phosphor plate 4 in the sub-scanning direction Y which is perpendicular to the main scanning direction X, and since the phosphor plate 4 is moved by the transfer means in the direction Y, two dimensional scanning is achieved by the laser beam over the phosphor plate 4.
Numeral 5 designates a condenser, numeral 6 a filter which has such filtering characteristics as shutting the laser beam off and enabling only the phosphor radiation to be passed, and numeral 7 a photo-multiplier. The electric signals of the radiation image information which has been read through the condenser, filter 6 and photo-multiplier 7 are converted into digital electric signals by way of a current/voltage (I/V) converter 8, an amplifier 9 and an analog/digital (A/D) converter 10. A digital output is provided at output terminal 11. Numeral 12 is a line start sensor, and numeral 13 designates a timing circuit. The start sensor 12 is adapted to generate a line start signal to the timing circuit 13 when the laser beam is detected at the sensor 12. The timing circuit 13 is thereby energized to output an operational timing pulse train to the A/D converter circuit 10, so that the irradiation by the excitation light in the main scanning direction X and the operation of the A/D converter 10 may be synchronized.
When the scanning of the pixels in the first line is completed and the phosphor plate 4 is moved in the sub-scanning direction Y, the pixels in the second line are scanned in the same way.
FIG. 2 shows a pattern of the spots of the laser beam which has been scanned over the phosphor plate by using the radiation image reading apparatus according to the prior art described above. More specifically, the size l.sub.X of respective pixels 41, 42, 43, . . . of the phosphor plate 4 in the main scanning direction X is decided depending on the sampling pitch in the A/D converter 10, and the size l.sub.Y of the pixels in the sub-scanning direction Y is decided by the shifting pitch of the phosphor plate 4 in the sub-scanning direction by the transferring means. Since the laser beam is deflected by the deflector 2 and continuously irradiated on the plate 4 in the main direction X, a plurality of spots of the laser beam would be generated in one pixel or such spots might be generated over the adjacent pixel in the main scanning direction X as shown in FIG. 2.
However, as is well known, the accelerated phosphor light has decay characteristics and scanning by the reading apparatus is of the so-called destructive scanning type which means the light emission from the phosphor plate will decrease in inverse proportion to the increase of the irradiation value of the excitation light. Therefore if the spots of the laser beam are continuously formed beyond a particular pixel to be read out at the current time as shown in FIG. 2, sharpness of the reproduced image in the main scanning direction will be rather inferior to that in the sub-scanning direction.